There are almost unlimited applications for the measurement of proportions of gases in a mixture. A particular example is rapid measurement of oxygen and carbon dioxide in the presence of nitrogen for evaluation of metabolic activity in humans or other organisms by indirect calorimetry. Other uses include general laboratory use, monitoring of combustion gases, and monitoring of green houses gases.
Commonly, to determine the proportions of N gases in a mixture, Nxe2x88x921 sensors are used, each one specific to a particular gas. This usually implies a multiplicity of instruments and sample paths, in many cases the necessity of making multiple adjustments to equalize delays and response times of the various instruments, the expense of procuring multiple instruments, and the inconvenience of dealing with them. An exception to this is the mass spectrograph, but this instrument is expensive, bulky, and complex. The present invention relates to the measurement of Nxe2x88x921 physical or chemical parameters in a mixture of N gases in order to determine the relative proportions of N gases. The measurements do not need to be specific to a particular gas, but they must be linearly independent in order to provide unique solutions for the proportions of all the gases.
In particular, the invention relates to the measurement of oxygen and carbon dioxide in a mixture of oxygen, carbon dioxide, and nitrogen, by the measurement of the paramagnetic properties of the gas and the speed of sound of the gas. Desirable qualities of gas measurement include long term stability without readjustment, fast response time, low temperature, non-expendable sensor, ruggedness, and low cost relative to current technologies.
Commonly, oxygen is measured by (1) classical paramagnetic method (Pauling method), (2) zirconium fuel cell, (3) magnetic wind method, (4) acoustical methods based on modulated magnetic fields, and (5) wet chemical cell such as the Clark electrode. (1) suffers from slow response and is easily damaged by physical shock. (2) suffers from poor stability, high temperature, need for warm up, and a consumable cell which must be replaced at intervals. (3) suffers from slow response, interference by other gases, and unstable calibration. (4) appears promising, but has not enjoyed significant commercial success, possibly due to poor stability and interference by acoustical noise. (5) suffers from slow speed, depletion, and changing calibration.
The most prevalent method of measuring carbon dioxide is the measurement of infrared absorption. Such instruments work fairly well, but require relatively high temperatures, significant warm up, and frequent calibration.
As already pointed out, the combination of two separate gas sensing technologies to determine the proportional combination of three gases is a deficient method as regards to complication, cost, matching of delay times and response times.
Methods in which the flux of a magnetic circuit is modulated by changing the composition of a gas in a gap have been proposed for years. These systems can be divided into two classes. In one class, the gas in a gap is displaced by a moving object such as a toothed wheel or a nitrogen filled chamber on a piezoelectric bender. Such systems are described by Hornfeck, U.S. Pat. No. 2,467,211; Greene U.S. Pat. No. 2,689,332; Sueda U.S. Pat. No. 3,720,870; and Delong et al U.S. Pat. No. 4,173,975. In the other class, a test gas and a reference gas are made to replace each other by pneumatic means, such as valving or a moving diaphragm. Hummel has described such methods in U.S. Pat. No. 3,049,665, and U.S. Pat. No. 4,683,426.
The difficulty with such systems lies in the extremely weak flux changes produced, which make the systems subject to interference by external magnetic fields and vibration, and to irreducible electrical noise due to resistance of the sensing coils. Dempster described a system based on magnetic bridge methods in U.S. Pat. No. 4,432,226, which to some degree mitigated these problems, but which still was marginal with respect to vibration, external fields, and electrical noise.
The use of speed of sound measurement to determine gas composition has been used to determine the combination of a binary gas mixture (Magori U.S. Pat. No. 5,325,703, Lura U.S. Pat. No. 5,351,592), but the inventor is unaware of prior art in which the speed of sound is used in conjunction with another physical or chemical measurement in order to determine the relative combinations of three gases in a mixture.
A difficulty with speed of sound sensing as an analytic method is that changes in speed may be relatively slight, but dependency on temperature is strong and apparatus may introduce its own variable components. Therefore, designing a system with the requisite long term stability is challenging. It is a feature of the invention that a reference gas and a sample gas are rapidly interchanged in the measuring system, a comparison made between the two gases under virtually identical conditions. This method results in a system of unparalleled stability.
It is an object of the present invention to:
1) Provide a method to determine the specific proportions of N gases in a mixture using Nxe2x88x921 sensors, at least one of which is not specific to any of the gases.
2) Provide a method for the measurement of oxygen and carbon dioxide in the presence of nitrogen, in a single integrated analyzer.
3) Provide a method for the measurement of oxygen in the presence of a wide variety of gases.
4) Provide a method for the measurement of respiratory gases with very fast response time making it suitable for breath by breath analysis.
5) Provide a method for the measurement of respiratory gases with exceptional stability and instant warm up.
6) Provide a method for the rapid measurement of respiratory gases without high temperature components, electrochemical cells, or consumable components.
Other objects, features and advantages of the present invention will become apparent from the following detailed description when taken in conjunction with the accompanying drawings.